It definately works with the prometheus. The first OS4 dev systems were A4ks with prometheus boards.
As we're talking A1200 here Mediator will definately work and I'm pretty sure G-REX will also be supported. I can see no logical reason why not. Try asking at AW. If the Freidens arn't too busy they may give you a definate answer on this one.
My advice would be to wait until the Powervixen arrives. The projected price looks to be less than a Blizz PPC + G-REX/Mediator, especially as it comes bundled with OS4. Also I believe the Powervixen should be able to function without an A1200, something to consider when running hardware which ended production in the 90's.
Provided availabity of AMY05/Powervixen boards. I'd imagine that all versions of OS4 will come at the same time.
